Season the chicken with 1 ¼ teaspoons of the salt, ¾ teaspoon of the garlic powder, and ¼ teaspoon of the pepper.
Lay 2 to 3 sage leaves over each chicken breast and toothpick into place.
Lightly coat the chicken in flour, tapping off any excess.
Heat the extra-virgin olive oil in a large skillet over medium heat, along with 1 tablespoon of the butter.
Working in batches to avoid crowding the pan, brown the chicken on each side (approx. 3 minutes).
Drain the chicken & set it aside.
Sprinkle 2 to 3 tablespoons of flour over the pan juices, and stir to until a thick paste forms.
Dribble in the Marsala, whisking out any lumps and pulling the brown bits up off the bottom of the pan.
Whisk in the chicken stock, and simmer the sauce until thickened (about 3 to 5 minutes).
Turn the heat to low and whisk in the remaining 3 tablespoons of butter, along with a few sage leaves, the remaining ¼ teaspoon of salt, ½ teaspoon of garlic powder, and ¼ teaspoon of black pepper.
Remove the toothpicks, then nestle the chicken back into the pan and baste with the pan sauce, cooking until it reaches an internal temperature of 175 degrees F.
Drape the chicken with prosciutto and top with fontina.
Place the lid over the pan and allow the cheese to melt.
Garnish with additional sage leaves and serve hot.